Feedstocks containing higher molecular weight hydrocarbons are cracked by contacting the feedstocks under elevated temperatures with a cracking catalyst whereby light and middle distillates are produced. Deterioration occurs in the cracking catalyst which can be partially attributable to the deposition on the catalyst of metals introduced into the cracking zone as contaminants in the feedstock. The deposition of these metals, such as nickel and vanadium, results in a decrease in the overall conversion of the feed as well as a decrease in the relative amount converted to gasoline. Another effect of these contaminant metals on the cracking catalyst is to catalyze dehydrogenation reactions, leading to an increased production of coke and hydrogen during the cracking process.
These catalyst poisoning metals are generally in organometallic form, such as in a porphyrin. During the catalytic cracking process, these metals deposit in a relatively non-volatile form on the cracking catalyst. These metal contaminants are generally specified as parts per million (ppm) nickel equivalents, defined as the sum of the nickel content in ppm plus one-fifth the vanadium content in ppm, plus one-tenth the iron content in ppm (nickel+0.2 vanadium+0.1 iron). As a general rule it is necessary to replace unprotected, contaminated catalyst with fresh catalyst at a rate sufficient to limit the amount of poisoning metals on the catalyst in order to prevent an excessive deterioration in catalyst performance.
U.S. Pat. No. 3,977,963 describes the passivation of a metal-poisoned cracking catalyst with bismuth; the use of an excess quantity of bismuth is illustrated in the example in which the bismuth to nickel equivalents weight ratio is 1.97:1.